Molecular fluorine laser with spectral linewidth of less than 1 pm

ABSTRACT

A narrow band molecular fluorine laser system includes an oscillator and an amplifier, wherein the oscillator produces a 157 nm beam having a linewidth less than 1 pm and the amplifier increases the power of the beam above a predetermined amount, such as more than one or several Watts. The oscillator includes a discharge chamber filled with a laser gas including molecular fluorine and a buffer gas, electrodes within the discharge chamber connected to a discharge circuit for energizing the molecular fluorine, and a resonator including the discharge chamber for generating a laser beam having a wavelength around 157 nm. Line-narrowing optics are included intra- and/or extra-resonator for reducing the linewidth of the laser beam to less than 1 pm. The amplifier may be the same or a different discharge chamber, and optical and/or electronic delays may be used for timing pulses from the oscillator to reach the amplifier at a maximum in the discharge current of the amplifier.

PRIORITY

This application claims the benefit of priority to U.S. provisionalpatent applications Ser. Nos. 60/140,531, filed Jun. 23, 1999,60/204,095, filed May 15, 2000, 60/162,735, filed Oct. 29, 1999,60/166,967, filed Nov. 23, 1999 and 60/170,342, filed Dec. 13, 1999.This application is a Continuation-in-Part of U.S. patent applicationSer. No. 09/317,527, filed May 24, 1999, which claims the benefit ofpriority to U.S. provisional patent applications Ser. No. 60/120,218,filed Feb. 12, 1999, and 60/119,486, filed Feb. 10, 1999. Thisapplication is also a Continuation-in-Part of U.S. patent applicationSer. No. 09/550,558, filed Apr. 17, 2000, which claims the benefit ofpriority to U.S. provisional patent application No. 60/130,392, filedApr. 19, 1999. All of the above priority applications are herebyincorporated by reference into the present application.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to a molecular fluorine laser systemincluding line-narrowing elements and method for generating a VUV laserbeam having a spectral linewidth of less than substantially 1 pm.

2. Description of the Related Art

Vacuum-UV microlithography takes advantage of the short wavelength ofthe molecular fluorine laser (157.6 nm), which allows the formation ofstructures of 0.1 μm or below by photolithographic exposure onsemiconductor substrates. TFT annealing and micro-machining applicationsmay also be performed advantageously at this wavelength.

Given the limited choice of high quality optical materials available inthis wavelength range for manufacturing imaging lenses, requirements ofminimal chromatic aberrations restrict spectral linewidths of the lasersource for refractive and partially achromatic imaging systems to below1 pm. The expectation is that spectral linewidths be between 0.1 pm and0.2 pm, and perhaps even below 0.1 pm in the future. Conventionalmolecular fluorine lasers emit VUV beams having spectral linewidths ofgreater than 1 pm.

A disadvantage of narrowing of spectral linewidth in a laser is that itcommonly leads to a significant decrease of efficiency and output power.Therefore, it is recognized in the present invention that to achieve adesired high throughput for 157 nm wafer steppers or wafer scanners, itwould be advantageous to have a line-narrowed molecular fluorine laseremitting an output beam of less than 1 pm, with a high output power thataverages anywhere from several watts to more than 10 watts.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to provide a VUVlaser system having-a narrow linewidth, i.e., less than substantially 1pm for producing small structures on silicon wafers.

It is a second object of the invention to provide a VUV laser having alinewidth of 1 pm or less which exhibits sufficient output power, i.e.,at least several Watts, to allow high throughput for VUV lithographyapplications at 157 nm.

Methods and apparatuses are provided in accord with the above objects,such as a narrow band molecular fluorine laser system including anoscillator and an amplifier, wherein the oscillator produces a 157 nmbeam having a linewidth less than 1 pm and the amplifier increases thepower of the beam above a predetermined amount, such as more than one orseveral Watts. The oscillator includes a discharge chamber filled with alaser gas including molecular fluorine and a buffer gas, electrodeswithin the discharge chamber connected to a discharge circuit forenergizing the molecular fluorine, and a resonator including thedischarge chamber and line-narrowing optics for generating the laserbeam having a wavelength around 157 nm and a linewidth less than 1 pm.

The amplifier preferably comprises a discharge chamber filled with alaser gas including molecular fluorine and a buffer gas, electrodesconnected to the same or a similar discharge circuit, e.g., using anelectrical delay circuit, for energizing the molecular fluorine. Theamplifier discharge is timed to be at or near a maximum in dischargecurrent when the pulse from the oscillator reaches the amplifierdischarge chamber.

The line-narrowing optics preferably include one or more etalons tunedfor maximum transmissivity of a selected portion of the spectraldistribution of the beam, and for relatively low transmissivity of outerportions of the spectral distribution of the beam. A prism beam expanderis preferably provided before the etalons for expanding the beamincident on the etalon or etalons. Two etalons may be used and tunedsuch that only a single interference order is selected.

The line-narrowing optics may further include a grating for selecting asingle interference order of the etalon or etalons corresponding to theselected portion of the spectral distribution of the beam. The resonatorfurther preferably includes an aperture within the resonator, andparticularly between the discharge chamber and the beam expander. Asecond aperture may be provided on the other side of the dischargechamber.

The line-narrowing optics may include no etalon. For example, the lineoptics may instead include only a beam expander and a diffractiongrating. The beam expander preferably includes two, three or four VUVtransparent prisms before the grating. The grating preferably has ahighly reflective surface for serving as a resonator reflector inaddition to its role of dispersing the beam.

The line-narrowing optics may include an etalon output coupler tuned formaximum reflectivity of a selected portion of the spectral distributionof the beam, and for relatively low reflectivity of outer portions ofthe spectral distribution of the beam. This system would also includeoptics such as a grating, dispersive prism or etalon, preferablyfollowing a beam expander, for selecting a single interference order ofthe etalon output coupler. The resonator would preferably have one ormore apertures for reducing stray light and divergence within theresonator.

In any of above configurations including a grating, a highly reflectivemirror may be disposed after the grating such that the grating and HRmirror form a Littman configuration. Alternatively, the grating mayserve to retroreflect as well as to dispserse the beam in a Littrowconfiguration. A transmission grating or grism may also be used.

The buffer gas preferably includes neon and/or helium for pressurizingthe gas mixture sufficiently to increase the output energy for a giveninput energy and to increase the energy stability, gas and tubelifetime, and/or pulse duration. The laser system further preferablyincludes a gas supply system for transferring molecular fluorine intodischarge chamber and thereby replenishing the molecular fluorine,therein, and a processor cooperating with the gas supply system tocontrol the molecular fluorine concentration within the dischargechamber to maintain the molecular fluorine concentration within apredetermined range of optimum performance of the laser.

The laser system may also include a spectral filter between theoscillator and the amplifier for further narrowing the linewidth of theoutput beam of the oscillator. The spectral filter may include an etalonor etalons following a beam expander. Alternatively, the spectral filtermay include a grating for dispersing and narrowing the beam. In thegrating embodiment, the spectral filter may include a lens focusing thebeam through a slit and onto a collimating optic prior to impinging uponthe beam expander-grating combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a molecular fluorine laser system inaccord with a preferred embodiment.

FIGS. 2a-2 f schematically show several alternative embodiments inaccord with a first aspect of the invention including various linenarrowing resonators and techniques utilizing line-narrowed oscillatorsfor the molecular fluorine laser.

FIG. 3a schematically shows a preferred embodiment in accord with asecond aspect of the invention including an oscillator, a spectralfilter in various configurations, and an amplifier.

FIGS. 3b-3 d schematically show alternative embodiments of spectralfilters in further accord with the second aspect of the invention.

FIG. 4a schematically shows an alternative embodiment in accord with thesecond aspect of the invention including a single discharge chamberproviding the gain medium for both an oscillator and an amplifier, and.having a spectral filter in between.

FIG. 4b(i)-(iii) respectively show waveforms of the electrical dischargecurrent, un-narrowed beam intensity and output beam intensity in accordwith the alternative embodiment of FIG. 3a.

FIG. 5a schematically shows a preferred embodiment in accord with athird aspect of the invention including a line-narrowed oscillatorfollowed by a power amplifier.

FIGS. 5b-5 f schematically show alternative embodiments of line-narrowedoscillators in further accord with the third aspect of the invention..

FIGS. 6a-6 b schematically show alternative embodiments in accord with afourth aspect of the invention including a single discharge chamberproviding the gain medium for both an oscillator with line-narrowing andan amplifier.

INCORPORATION BY REFERENCE

What follows is a cite list of references each of which is, in additionto those references cited above in the priority section, herebyincorporated by reference into the detailed description of the preferredembodiment below, as disclosing alternative embodiments of elements orfeatures of the preferred embodiments. A single one or a combination oftwo or more of these references may be consulted to obtain a variationof the preferred embodiments described in the detailed descriptionbelow. Further patent, patent application and non-patent references arecited in the written description and are also incorporated by referenceinto the preferred embodiment with the same effect as just describedwith respect to the following references:

1- U. Stamm, “Status of 157 nm The 157 Excimer Laser” InternationalSEMATECH 157 nm Workshop, Feb. 15-17, 1999, Litchfield, Ariz., USA;

2- T. Hofman, J. M. Hueber, P. Das, S. Scholler, “Prospects of HighRepetition Rate F₂ (157 nm) Laser for Microlithography”, InternationalSEMATECH 157 Workshop, Feb. 15-17, 1999, Litchfield, Ariz., USA;

3- U. Stamm, I. Bragin, S. Govorkov, J. Kleinschmidt, R. Patzel, E.Slobodtchikov, K. Vogler, F. Voss, and D. Basting, “Excimer Laser for157 nm Lithography”, 24^(th) International Symposium onMicrolithography, Mar. 14-19, 1999, Santa Clara, Calif., USA;

4- T. Hofmann, J. M. Hueber, P. Das, S. Scholler, “Revisiting The F₂Laser For DUV microlithography”, 24^(th) International Symposium onMicrolithography, Mar. 14-19, 1999, Santa Clara, Calif., USA.

5- W. Muckenheim, B. Ruckle, “Excimer Laser with Narrow Linewidth andLarge Internal Beam Divergence”, J. Phys. E: Sci. Instrum. 20 (1987)1394;

6- G. Grunefeld, H. Schluter, P. Andersen, E. W. Rothe, “Operation ofKrF and ArF Tunable Excimer Lasers Without Cassegrain Optics”, AppliedPhysics B 62 (1996) 241;

7- U.S. patent application Ser. Nos. 09/317,526, 09/343,333, 60/122,145,60/140,531, 60/162,735, 60/166,952, 60/171,172, 60/141,678, 60/173,993,60/166,967, 60/172,674, and 60/181,156, and U.S. patent application ofKleinschmidt, serial number not yet assigned, filed May 18, 2000, for“Reduction of Laser Speckle in Photolithography by Controlled Disruptionof Spatial Coherence of Laser Beam,” and U.S. Pat. No. 6,005,880, eachof which is assigned to the same assignee as the present application;and

8- W. Mueckenheim, “Seven Ways to Combine Two Excimer Lasers,” reprintedfrom July 1987 edition of Laser Focus/Electro-Optics.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a VUV laser system, preferably a molecular fluorinelaser for deep ultraviolet (DUV) or vacuum ultraviolet (VUV)lithography, is schematically shown. Alternative configurations forlaser systems for use in such other industrial applications as TFTannealing and/or micromachining, e.g., are understood by one skilled inthe art as being similar to and/or modified from the system shown inFIG. 1 to meet the requirements of that application. For this purpose,alternative VUV laser system and component configurations are describedat U.S. patent application Ser. Nos. 09/317,695, 09/317,526, 09/317,527,09/343,333, 60/122,145, 60/140,531, 60/162,735, 60/166,952, 60/171,172,60/141,678, 60/173,993, 60/166,967, 60/172,674, and 60/181,156, and U.S.patent application of Kleinschmidt, serial number not yet assigned,filed May 18, 2000, for “Reduction of Laser Speckle in Photolithographyby Controlled Disruption of Spatial Coherence of Laser Beam,” and U.S.Pat. No. 6,005,880, each of which is assigned to the same assignee asthe present application and is hereby incorporated by reference.

The system shown in FIG. 1 generally includes a laser chamber 2 having apair or several pairs of main discharge electrodes 3 connected with asolid-state pulser module 4, and a gas handling module 6. Thesolid-state pulser module 4 is powered by a high voltage power supply 8.The laser chamber 2 is surrounded by optics module 10 and optics module12, forming a resonator. The optics modules 10 and 12 are controlled byan optics control module 14.

A computer 16 for laser control receives various inputs and controlsvarious operating parameters of the system. A diagnostic module 18receives and measures various parameters of a split off portion of themain beam 20 via optics for deflecting a small portion of the beamtoward the module 18, such as preferably a beam splitter module 21, asshown. The beam 20 is preferably the laser output to an imaging system(not shown) and ultimately to a workpiece (also not shown). The lasercontrol computer 16 communicates through an interface 24 with astepper/scanner computer 26 and other control units 28.

The laser chamber 2 contains a laser gas mixture and includes a pair ofor several pairs of main discharge electrodes 3 and one or morepreionization electrodes (not shown). Preferred main electrodes 3 aredescribed at U.S. patent application Ser. Nos. 09/453,670, 60/184,705and 60/128,227, each of which is assigned to the same assignee as thepresent application and is hereby incorporated by reference. Otherelectrode configurations are set forth at U.S. Pat. Nos. 5,729,565 and4,860,300, each of which is assigned to the same assignee, andalternative embodiments are set forth at U.S. Pat. Nos. 4,691,322,5,535,233 and 5,557,629, all of which are hereby incorporated byreference. The laser chamber 2 also includes a preionization arrangement(not shown). Preferred preionization units are set forth at U.S. patentapplication Ser. Nos. 60,162,845, 60/160,182, 60/127,237, 09/535,276 and09/247,887, each of which is assigned to the same assignee as thepresent application, and alternative embodiments are set forth at U.S.Pat. Nos. 5,337,330, 5,818,865 and 5,991,324, all of the above patentsand patent applications being hereby incorporated by reference.

The solid-state pulser module 14 and high voltage power supply 8 supplyelectrical energy in compressed electrical pulses to the preionizationand main electrodes 3 within the laser chamber 2 to energize the gasmixture. The preferred pulser module and high voltage power supply aredescribed at U.S. patent application Ser. Nos. 60/149,392, 60/198,058,and 09/390,146, and U.S. patent application of Osmanow, et al., serialnumber not yet assigned, filed May 15, 2000, for “Electrical ExcitationCircuit for Pulsed Laser”, and U.S. Pat. Nos. 6,005,880 and 6,020,723,each of which is assigned to the same assignee as the presentapplication and which is hereby incorporated by reference into thepresent application. Other alternative pulser modules are described atU.S. Pat. Nos. 5,982,800, 5,982,795, 5,940,421, 5,914,974, 5,949,806,5,936,988, 6,028,872 and 5,729,562, each of which is hereby incorporatedby reference. A conventional pulser module may generate electricalpulses in excess of 3 Joules of electrical power (see the '988 patent,mentioned above).

The laser resonator which surrounds the laser chamber 2 containing thelaser gas mixture includes optics module 10 including line-narrowingoptics for a line narrowed excimer or molecular fluorine laser, whichmay be replaced by a high reflectivity mirror or the like in a lasersystem wherein either line-narrowing is not desired, or if linenarrowing is performed at the front optics module 12, or an spectralfilter external to the resonator is used for narrowing the linewidth ofthe output beam. Several variations of line-narrowing optics are setforth in detail below.

The laser chamber 2 is sealed by windows transparent to the wavelengthsof the emitted laser radiation 14. The windows may be Brewster windowsor may be aligned at another angle to the optical path of the resonatingbeam. The beam path between the laser chamber and each of the opticsmodules 10 and 12 is sealed by enclosures 17 and 19, and the interiorsof the enclosures is substantially free of water vapor, oxygen,hydrocarbons, fluorocarbons and the like which otherwise strongly absorbVUV laser radiation.

After a portion of the output beam 20 passes the outcoupler of theoptics module 12, that output portion impinges upon beam splitter module21 which includes optics for deflecting a portion of the beam to thediagnostic module 18, or otherwise allowing a small portion of theoutcoupled beam to reach the diagnostic module 18, while a main beamportion 20 is allowed to continue as the output beam 20 of the lasersystem. Preferred optics include a beamsplitter or otherwise partiallyreflecting surface optic. The optics may also include a mirror or beamsplitter as a second reflecting optic. More than one beam splitterand/or HR mirror(s), and/or dichroic mirror(s) may be used to directportions of the beam to components of the diagnostic module 18. Aholographic beam sampler, transmission grating, partially transmissivereflection diffraction grating, grism, prism or other refractive,dispersive and/or transmissive optic or optics may also be used toseparate a small beam portion 22 from the main beam 20 for detection atthe diagnostic module 18, while allowing most of the main beam 20 toreach an application process directly or via an imaging system orotherwise. The output beam 20 may be transmitted at the beam splittermodule while a reflected beam portion 22 is directed at the diagnosticmodule 18, or the main beam 20 may be reflected, while a small portion22 is transmitted to the diagnostic module 18. The portion of theoutcoupled beam which continues past the beam splitter module 21 is theoutput beam 20 of the laser, which propagates toward an industrial orexperimental application such as an imaging system and workpiece forphotolithographic applications.

An enclosure 23 seals the beam path of the beams 22 and 20 such as tokeep the beam paths free of photoabsorbing species. Smaller enclosures17 and 19 seal the beam path between the chamber 2 and the opticsmodules 10 and 12. The preferred enclosure 23 and beam splitting module21 are described in detail in the Ser. No, 09/343,333 and 60/140,530applications, incorporated by reference above, and in U.S. patentapplication Ser. No. 09/131,580, which is assigned to the same assigneeand U.S. Pat. Nos. 5,559,584, 5,221,823, 5,763,855, 5,811,753 and4,616,908, all of which are hereby incorporated by reference. Forexample, the beam splitting module 21 preferably also includes opticsfor filtering visible red light from the beam 22 so that substantiallyonly VUV light is received at a detector of the diagnostic module 18.Filtering optics may also be included for filtering red light from theoutput beam 20. Also, an inert gas purge is preferably flowing throughthe enclosure 23.

The diagnostic module 18 preferably includes at least one energydetector. This detector measures the total energy of the beam portionthat corresponds directly to the energy of the output beam 20. Anoptical configuration such as an optical attenuator, e.g., a plate or acoating, or other optics may be formed on or near the detector or beamsplitter module 21 to control the intensity, spectral distributionand/or other parameters of the radiation impinging upon the detector(see U.S. patent application Ser. Nos. 09/172,805, 60/172,749,60/166,952 and 60/178,620, each of which is assigned to the sameassignee as the present application and is hereby incorporated byreference).

One other component of the diagnostic module 18 is preferably awavelength and/or bandwidth detection component such as a monitor etalonor grating spectrometer (see U.S. patent application Ser. Nos.09/416,344, 60/186,003, 60/158,808, and 60/186,096, and Lokai, et al.,serial number not yet assigned, “Absolute Wavelength Calibration ofLithography Laser Using Multiple Element or Tandem See Through HollowCathode Lamp”, filed May 10, 2000, each of which is assigned to the sameassignee as the present application, and U.S. Pat. Nos. 4,905,243,5,978,391, 5,450,207, 4,926,428, 5,748,346, 5,025,445, and 5,978,394,all of the above wavelength and/or bandwidth detection and monitoringcomponents being hereby incorporated by reference.

Other components of the diagnostic module may include a pulse shapedetector or ASE detector, such as are described at U.S. patentapplication Ser. Nos. 09/484,818 and 09/418,052, respectively, each ofwhich is assigned to the same assignee as the present application and ishereby incorporated by reference, such as for gas control and/or outputbeam energy stabilization. There may be a beam alignment monitor, e.g.,such as is described at U.S. Pat. No. 6,014,206 which is herebyincorporated by reference.

The processor or control computer 16 receives and processes values ofsome of the pulse shape, energy, amplified spontaneous emission (ASE),energy stability, energy overshoot for burst mode operation, wavelength,spectral purity and/or bandwidth, among other input or output parametersof the laser system and output beam. The processor 16 also controls theline narrowing module to tune the wavelength and/or bandwidth orspectral purity, and controls the power supply and pulser module 4 and 8to control preferably the moving average pulse power or energy, suchthat the energy dose at points on the workpiece is stabilized around adesired value. In addition, the computer 16 controls the gas handlingmodule 6 which includes gas supply valves connected to various gassources.

The laser gas mixture is initially filled into the laser chamber 2during new fills. The gas composition for a very stable excimer laser inaccord with the preferred embodiment uses helium or neon or a mixture ofhelium and neon as buffer gas, depending on the laser. Preferred gascomposition are described at U.S. Pat. Nos. 4,393,405 and 4,977,573 andU.S. patent application Ser. Nos. 09/317,526, 09/513,025, 60/124,785,09/418,052, 60/159,525 and 60/160,126, each of which is assigned to thesame assignee and is hereby incorporated by reference into the presentapplication. The concentration of the fluorine in the gas mixture mayrange from 0.003% to 1.00%, and is preferably around 0.1%. An additionalgas additive, such as a rare gas, may be added for increased energystability and/or as an attenuator as described in the '025 application,mentioned above. Specifically, for the F2-laser, an addition of Xenonand/or Argon may be used. The concentration of xenon or argon in themixture may range from 0.0001% to 0.1%. For an ArF-laser, an addition ofxenon or krypton may be used also having a concentration between 0.0001%to 0.1%.

Halogen and rare gas injections, total pressure adjustments and gasreplacement procedures are performed using the gas handling module 6preferably including a vacuum pump, a valve network and one or more gascompartments. The gas handling module 6 receives gas via gas linesconnected to gas containers, tanks, canisters and/or bottles. Preferredgas handling and/or replenishment procedures of the preferredembodiment, other than as specifically described herein, are describedat U.S. Pat. Nos. 4,977,573 and 5,396,514 and U.S. patent applicationSer. Nos. 60/124,785, 09/418,052, 09/379,034, 60/171,717, and60/159,525, each of which is assigned to the same assignee as thepresent application, and U.S. Pat. Nos. 5,978,406, 6,014,398 and6,028,880, all of which are hereby incorporated by reference. A Xe gassupply may be included either internal or external to the laser systemaccording to the '025 application, mentioned above.

A general description of the line-narrowing features of the severalembodiments of the present is first provided here, followed by adetailed discussion referring FIGS. 2a-6 b. Exemplary line-narrowingoptics are contained in the optics module 10 include a beam expander, anoptional etalon and a diffraction grating, which produces a relativelyhigh degree of dispersion, for a narrow band laser such as is used witha refractive or catadioptric optical lithography imaging system. Asmentioned above, the front optics module may include line-narrowingoptics as well (see the Ser. Nos. 60/166,277, 60/173,993 and 60/166,967applications, each being assigned to the same assignee and herebyincorporated by reference). For a semi-narrow band laser such as is usedwith an all-reflective imaging system, and which is not the subject ofthe present invention, the grating is replaced with a highly reflectivemirror, and a lower degree of dispersion may be-produced by a dispersiveprism. A semi-narrow band laser would typically have an output beamlinewidth in excess of 1 pm and may be as high as 100 pm in some lasersystems, depending on the characteristic free-running bandwidth of thelaser.

The beam expander of the above exemplary line-narrowing optics of theoptics module 10 preferably includes one or more prisms. The beamexpander may include other beam expanding optics such as a lens assemblyor a converging/diverging lens pair. The grating or highly reflectivemirror is preferably rotatable so that the wavelengths reflected intothe acceptance angle of the resonator can be selected or tuned.Alternatively, the grating, or other optic or optics, or the entireline-narrowing module may be pressure tuned, such as it set forth in theSer. No. 60/178,445 and 09/317,527 applications, each of which isassigned to the same assignee and is hereby incorporated by reference.The grating may be used both for dispersing the beam for achievingnarrow bandwidths and also preferably for retroreflecting the beam backtoward the laser tube. Alternatively, a highly reflective mirror ispositioned after the grating which receives a reflection from thegrating and reflects the beam back toward the grating to doubly dispersethe beam, or the grating may be a transmission grating. One or moredispersive prisms may also be used, and more than one etalon may beused.

Depending on the type and extent of line-narrowing and/or selection andtuning that is desired, and the particular laser that the line-narrowingoptics are to be installed into, there are many alternative opticalconfigurations that may be used. For this purpose, those shown in U.S.Pat. Nos. 4,399,540, 4,905,243, 5,226,050, 5,559,816, 5,659,419,5,663,973, 5,761,236, and 5,946,337, and U.S. patent application Ser.Nos. 09/317,695, 09/130,277, 09/244,554, 09/317,527, 09/073,070,60/124,241, 60/140,532, 60/147,219 and 60/140,531, 60/147,219,60/170,342, 60/172,749, 60/178,620, 60/173,993, 60/166,277, 60/166,967,60/167,835, 60/170,919, 60/186,096, each of which is assigned to thesame assignee as the present application, and U.S. Pat. Nos. 5,095,492,5,684,822, 5,835,520, 5,852,627, 5,856,991, 5,898,725, 5,901,163,5,917,849, 5,970,082, 5,404,366, 4,975,919, 5,142,543, 5,596,596,5,802,094, 4,856,018, 5,970,082, 5,978,409, 5,999,318, 5,150,370 and4,829,536, and German patent DE 298 22 090.3, are each herebyincorporated by reference into the present application.

Optics module 12 preferably includes means for outcoupling the beam 20,such as a partially reflective resonator reflector. The beam 20 may beotherwise outcoupled such as by an intraresonator beam splitter orpartially reflecting surface of another optical element, and the opticsmodule 12 would in this case include a highly reflective mirror. Theoptics control module 14 controls the optics modules 10 and 12 such asby receiving and interpreting signals from the processor 16, andinitiating realignment or reconfiguration procedures (see the '241,'695, 277, 554, and 527 applications mentioned above).

A detailed discussion of the line-narrowing configurations of anoscillator element of the laser system according to the preferredembodiment is now set forth with reference to FIGS. 2a-2 f. Severalembodiments of an oscillator of the laser system using line-narrowingtechniques for the molecular fluorine laser, are shown in FIGS. 2a-2 fto meet or substantially meet the first object of the invention.

FIG. 2a schematically shows an oscillator of a laser system according toa first embodiment including a discharge chamber 2 preferably containingmolecular fluorine and a buffer gas of neon, helium or a combinationthereof (see the Ser. No. 09/317,526 application), and having a pair ofmain discharge electrodes 3 (not shown) and a preionization arrangement(also not shown) therein. The system shown in FIG. 2a also includes aprism beam expander 30 and a diffraction grating 32 arranged in aLittrow configuration. The beam expander 30 may include one or moreprisms and preferably includes several prisms. The beam expander servesto reduce divergence of the beam incident onto the grating, thusimproving wavelength resolution of the wavelength selector. The gratingis preferably a high blaze angle echelle grating (see the Ser. No.60/170,342 application incorporated by reference above).

The system shown includes a pair of apertures 34 in the resonator whichreject stray light and reduce broadband background, and can also serveto reduce the linewidth of the beam by lowering the acceptance angle ofthe resonator. Alternatively, one aperture 34 on either side of thechamber 2 may be included, or no apertures 34 may be included. Exemplaryapertures 34 are set forth at U.S. Pat. No. 5,161,238, which is assignedto the same assignee and is hereby incorporated by reference (see alsothe Ser. No. 09/130,277 application incorporated by reference above).

The system of FIG. 2a also includes a partially reflecting outputcoupling mirror 36. The outcoupling mirror 36 may be replaced with ahighly reflective mirror, and the beam may be otherwise output coupledsuch as by using a polarization reflector or other optical surfacewithin the resonator such as a surface of a prism, window orbeam-splitter (see, e.g., U.S. Pat. No. 5,150,370, incorporated byreference above).

The system shown at FIG. 2b includes the chamber 2, the apertures 34,the partially reflecting output coupling mirror 36 and beam expander 30described above with respect to FIG. 2a. The system of FIG. 2b alsoincludes a diffraction grating 38 and a highly reflective mirror 40. Thegrating 38 preferably differs from the grating 32 of FIG. 2a either inits orientation with respect to the beam, or its configuration such asits blaze angle, etc., or both. The laser beam is incident onto thegrating 38 at an angle closer to 90° than for the grating 32. Theincidence angle is, in fact, preferably very close to 90°. This isarrangement is referred to here as the Littman configuration. TheLittman configuration increases the wavelength dispersion of the grating38. After passing through or reflecting from the diffraction grating 38,the diffracted beam is reflected by the highly reflective mirror 40. Thetuning of the wavelength is preferably achieved by tilting the highlyreflective mirror 40. As mentioned above with respect to the exemplaryarrangement, tuning may be achieved otherwise by rotating another opticor by pressure tuning one or more optics, or otherwise as may beunderstood by one skilled in the art.

FIG. 2c schematically shows another embodiment of an oscillator having alaser chamber 2, apertures 34, outcoupler 36, beam expander 30 andLittrow diffraction grating 32, preferably as described above. Inaddition, the system of FIG. 2c includes one or more etalons 42, e.g.,two etalons are shown, which provide high-resolution line narrowing,while the grating 32 serves to select a single interference order of theetalons 42. The etalon or etalons 42 may be placed in various positionsin the resonator, i.e., other than as shown. For example, a prism orprisms of the beam expander 30 may be positioned between an etalon oretalons 42 and the grating. An etalon 42 may be used as an outputcoupler, as will be described in more detail below with reference toFIGS. 2e-2 f. The arrangement of FIG. 2c (as well as FIG. 2d below)including an etalon or etalons 42 may be varied as described at any ofU.S. patent application Ser. Nos. 60/162,735, 60/178,445, or 60/158,808,each of which is assigned to the same assignee and is herebyincorporated by reference.

FIG. 2d shows another embodiment of the laser system having one or moreetalons 43, e.g., two etalons 43 are shown. The system of FIG. 2d is thesame as that of FIG. 2c except that the grating 32 is replaced with ahighly reflective mirror, and the etalons 43 are differently configuredowing to the omission of the grating 32 which is not available, as inthe system of FIG. 2c, to select a single interference order of theetalons 43. The free spectral ranges of etalons 43 are instead adjustedin such a way that one of the etalons 43, preferably the first etalon 43after the beam expander 30, selects a single order of the other etalon43, e.g., the second etalon 43. The second etalon 43 of the preferredarrangement is, therefore, allowed to have a smaller free spectral rangeand higher wavelength resolution. Some further alternative variations ofthe etalons 43 of the system of FIG. 2d may be used as set forth in U.S.Pat. No. 4,856,018, which is hereby incorporated by. reference.

FIGS. 2e and 2 f schematically show embodiments similar to thearrangements described above with reference to FIGS. 2a and 2 b,respectively, which differ in that the partially reflecting outcouplermirror 36 is replaced with a reflective etalon outcoupler 46. The etalonoutcoupler 46 is used in combination with the grating 32 or 38 and beamexpander 30 of FIGS. 2e and 2 f, respectively, wherein the grating 32 or38 selects a single interference order of the etalon outcoupler 46.Alternatively, one or more dispersive prisms or another etalon may beused in combination with the etalon outcoupler 46 for selecting a singleinterference order of the etalon 46. The grating 32 or 38 restrictswavelength range to a single interference order of the outcoupler etalon46. Variations of the systems of FIGS. 2e and 2 f that may be used incombination with the systems set forth at FIGS. 2e and/or 2 f are setforth at the Ser. No. 09/317,527 and 60/166,277 applications,incorporated by reference above, and U.S. Pat. Nos. 6,028,879,3,609,586, 3,471,800, 3,546,622, 5,901,163, 5,856,991, 5,440,574, and5,479,431, and H. Lengfeliner, Generation of tunable pulsed microwaveradiation by nonlinear interaction of Nd:YAG laser radiation in GaPcrystals, Optics Letters, Vol. 12, No. 3 (March 1987), S. Marcus, Cavitydumping and coupling modulation of an etalon-coupled CO₂ laser, J. AppI.Phys., Vol. 53, No. 9 (September 1982), and The physics and technologyof laser resonators, eds. D. R. Hall and P. E. Jackson, at p. 244, eachof which is hereby incorporated by reference.

In all of the above embodiments shown and described with reference toFIGS. 2a-2 f, the. material used for the prisms of the beam expanders30, etalons 42, 43, 46 and laser windows is preferably one that ishighly transparent at wavelengths below 200 nm, such as at the 157 nmoutput emission wavelength of the molecular fluorine laser. Thematerials are also capable of withstanding long-term exposure toultraviolet light with minimal degradation effects. Examples of suchmaterials are CaF₂, MgF₂, BaF, BaF₂, LiF, LiF₂, and SrF₂. Also, in allof the above embodiments of FIGS. 2a-2 f, many optical surfaces,particularly those of the prisms, preferably have, an anti-reflectivecoating on one or more optical surfaces, in order to minimize reflectionlosses and prolong their lifetime.

Also, as mentioned in the general description above, the gas compositionfor the F₂ laser in the above configurations uses either helium, neon,or a mixture of helium and neon as a buffer gas. The concentration offluorine in the buffer gas preferably ranges from 0.003% to around 1.0%,and is preferably around 0.1%. The addition of a trace amount of xenon,and/or argon, and /or oxygen, and/or krypton and/or other gases may beused for increasing the energy stability, burst control, or outputenergy of the laser beam. The concentration of xenon, argon, oxygen, orkrypton in the mixture may range from 0.0001% to 0.1%. Some alternativegas configurations including trace gas additives are set forth at U.S.patent application Ser. Nos. 09/513,025 and 09/317,526, each of which isassigned to the same assignee and is hereby incorporated by reference.

All of the oscillator configurations shown above at FIGS. 2a-2 f may beadvantageously used to produce a VUV beam 20 having a wavelength ofaround 157 nm and a linewidth of around 1 pm or less. Some of thoseconfigurations having an output linewidth of less than 1 pm already meetthe above first object of the invention with respect to the linewidth.Those oscillators may be used with other elements, such as an amplifier,as set forth below at FIGS. 3a-6 b to meet the second object of theinvention, i.e., to achieve sufficient output power for substantialthroughput at a 157 nm lithography fab. Other oscillators producinglinewidths above 1 pm may be advantageously used in combination withother line-narrowing elements such as a spectral filter, as set forthbelow at FIGS. 3a-4 b, to meet that first object, and with an amplifieras set forth in the embodiments of FIGS. 3a-4 b to meet the secondobject.

FIG. 3a schematically illustrates, in block form, a laser system inaccord with a preferred embodiment of the present invention, wherein anarrower linewidth is desired than is output by the oscillator 48, andhigher power is desired than is output by the oscillator 48. To reducethe linewidth, the output beam 20 of the oscillator 48 is directedthrough a spectral filter 50. To increase the output power,. the beam 20is directed through an amplifier 52.

The system of FIG. 3a includes a line-narrowed oscillator 48, a spectralfilter 50 and an amplifier 52. Various preferred configurations of thespectral filter 50 are described below with reference to FIGS. 3b-3 d.The oscillator 48 of FIG. 3a is an electrical discharge molecularfluorine laser producing a spectral linewidth of approximately 1 pm, andis preferably one of the configurations described above with respect toFIGS. 2a-2 f, or a variation thereof as described above, or as may beunderstood as being advantageous to one skilled in the art, such as maybe found in one or more of the reference incorporated by referenceabove. The oscillator 48 is followed by the spectral filter 50, whichtransmits light in a narrower spectral range, i.e., less than thelinewidth of the output beam 20 from the oscillator or less than around1 pm. Lastly, the transmitted beam is amplified in an amplifier 52 basedon a separate discharge chamber to yield an output beam 54 that meetsboth the first and second objects of the invention. Preferably, theoscillator and amplifier discharges are synchronized using a delaycircuit and advantageous solid-state pulser circuit such as is describedat U.S. patent application Ser. No. 60/204,095 and at U.S. Pat. Ser. No.6,005,880, each of which is assigned to the same assignee and is herebyincorporated by reference.

The spectral filter 50 is preferably includes one of the arrangementsshown in FIGS. 3b-3 d. Variations may be understood as advantageous toone skilled in the art using any of a large number of combinations ofprisms, gratings, grisms, holographic beam samplers, etalons, lenses,apertures, beam expanders, collimating. optics, etc., for narrowing thelinewidth of the input beam 20, preferably without consuming asubstantial fraction of the energy of the input beam 20.

FIG. 3b illustrates a first spectral filter 50 embodiment including abeam expander followed by one or more etalons 58 to yield an output beamhaving a linewidth substantially below the linewidth, e.g., around 1 pm,of the input beam 20 to meet the first object of the invention. Eachetalon 58 includes two partially reflecting surfaces of reflectivity R,separated by a preferably gas-filled gap of thickness D. Thetransmission spectrum of the etalon T(λ) is described by a periodicfunction of the wavelength λ:

T(λ)=(1+(4F ²/π²)sin(2πnDcos(Θ))/λ))⁻¹  (1)

where n is the refractive index of the material, preferably an inertgas, filling the etalon 58, Θ is the tilt angle of the etalon 58 withrespect to the beam, and F is the finesse of the etalon 58 which isdefined as:

F=πR ^(½)/(1−R)  (2)

The reflectivity R and spacing of the etalon D can be selected in such away that only a single transmission maximum overlaps with the emissionspectrum of the broader-band oscillator 48. For instance, if the finesseof the etalon 58 is selected to be 10, then the spectral width of thetransmission maximum is roughly {fraction (1/10)} of the free spectralrange (FSR) of the etalon 58. Therefore, selecting a free spectral rangeof 1 pm will produce a transmitted beam with spectral linewidth of 0.1pm, without sidebands since the linewidth of the oscillator (48) output(approximately 1 pm) is significantly less than two times the FSR.

Using multiple etalons 58 allows a higher contrast ratio, which isdefined as a ratio of the maximum transmission to the transmission ofthe wavelength halfway between the maxima. This contrast ratio for asingle etalon is approximately equal to (1+4F²/π²). Higher finessevalues lead to higher contrast. For several etalons 58, the totalcontrast ratio will be (1+4F²/π²)^(n) where n is the number of etalons58 used. Additionally, the spectral width of the transmission maximawill be reduced with increased number of etalons 58 used. Disadvantagesof using several etalons 58 include high cost and complexity of theapparatus and increased optical losses.

The beam expander 56 shown at FIG. 3b serves to reduce the divergence ofthe beam incident onto the etalons 58. From the formula (1), it followsthat a change in the beam incidence angle Θ causes a shift of thewavelength at which maximum transmission occurs. Assuming an FSR of 1pm, the etalon spacing is D=1.2 cm. If the transmission interferencespectrum of the etalon 58 is at its maximum at normal incidence (Θ=0),then the angle Θ, at which the transmission spectrum reaches maximumagain is Θ˜(λ/nD)^(½)=3.6 mrad. Therefore, it is preferred that thespectral filter 50 shown at FIG. 3b be configured such that thedivergence of the beam is below Θ, and preferably by a factor comparableto the finesse F of the etalon 58. Since the divergence of a typicalmolecular fluorine laser is several millirads, the advantage of usingthe beam expander 56 to reduce this divergence from typically above Θ asit is output from the oscillator 48 to below Θ, is may be understood. Itis also preferred to use one or more apertures 34 in the oscillator 48to reduce its output divergence (see the Ser. No. 09/130,277application, mentioned above).

The gaps between the plates of the etalons 58 are preferably filled withan inert gas. Tuning of the transmitted wavelength can be accomplishedby changing the pressure of the gas as described in the Ser. No.09/317,527 application, mentioned above. In addition to pressure tuningand rotation tuning of the etalon's output transmission spectrum, theetalons 58 may be piezoelectrically tuned such as to geometrically alterthe gap spacing.

FIG. 3c schematically illustrates a second embodiment of the spectralfilter 50 of FIG. 3a generally utilizing a diffraction grating 60.Although there are other ways to configure the spectral filter 50according to the second embodiment using a grating 60, an example isshown at FIG. 3c and described here. The spectral filter 50 shown atFIG. 3c is a Czerny-Turner type: spectrometer, modified to achieve highresolution. The input beam 20 in focused by a lens 61 a through an inputslit 62 a after which the beam is incident on a collimating mirror 64.After reflection from the mirror 64, the beam is incident on a beamexpander 66 and then onto the grating 60. The beam is dispersed andreflected from the grating 60, after which the beam retraverses the beamexpander 66, and is reflected from the collimating mirror 64 through anoutput slit 62 b at or near the focal point of a lens 62 b. The outputbeam 59 then has a linewidth substantially less than the linewidth,e.g., around 1 pm, of the input beam 20, or substantially less than 1 pmto meet the first object of the invention.

The diffraction grating 60 is preferably a high blaze echelle grating60. The wavelength dispersion of this preferred grating 60 is describedby the following formula:

d/λ/dΘ=(2/λ)tanΘ  (3)

where Θ is the incidence angle. The spectral width Δλ of the transmittedbeam is determined by the dispersion dλ/dΘ of the grating 60, themagnification factor M of the prism expander 66, the focal length L ofthe collimating mirror 64 and the width d of the slits 62 a, 62 b of thespectrometer:

Δλ=d(L M dλ/dΘ)⁻¹  (4)

For example, using an echelle grating 60 wherein the incidence angle Θis 78.6°, L=2 m and M=8, the slit width d which would achieve 0.1 pmresolution for the spectral filter 50 of FIG. 3c is around d=0.1 mm. Itis preferred, therefore, to reduce the divergence of the oscillator 48in order to increase the transmission of the beam 20 through the inputslit 61 a. This can be advantageously achieved by using apertures insidethe resonator of the oscillator 48 (see again the Ser. No. 09/130,277application, mentioned above).

The third example of a spectral filter 50 that may be used inillustrated at FIG. 3d. The spectral filter 50 of FIG. 3d differs fromthat shown at FIG. 3c in that a collimating lens 68 is used in theembodiment of FIG. 3d, rather than a collimating mirror 64, as is usedin the embodiment of FIG. 3c. An advantage of the embodiment of FIG. 3dis its simplicity and the absence of astigmatism introduced by themirror 64 of FIG. 3c at non-zero incidence angle.

It is useful to reiterate here that synchronization of the electricaldischarge pulses in the chambers 2 of the oscillator 48 and amplifier 52is preferred in order to ensure that the line-narrowed optical pulsefrom the oscillator 48 arrives at the chamber 2 of the amplifier 52 atthe instance when the gain of the amplifier 52 is at or near itsmaximum. Additionally, this preferred synchronization timing should bereproducible from pulse to pulse to provide high energy stability of theoutput pulses. The preferred embodiment electronic circuitry allowingthis precise timing control is described at U.S. Pat. No. 6,005,880 andU.S. patent application Ser. No. 60/204,095, as mentioned above.

FIG. 4a shows the use of a single discharge chamber 70 that provides thegain medium for both an oscillator and an amplifier. The setup of FIG.4a includes the discharge chamber 70 within a resonator including ahighly reflective mirror 72 and a partially reflecting outcouplingmirror 74. A pair of apertures 34 are also included, as described above,to match the divergence of the resonator of this oscillator 48. A smallportion of the cross-section of the discharge volume is used to producean un-narrowed beam 76 with this oscillator configuration. It is alsopossible to include one or more line-narrowing components with thisoscillator configuration, or to otherwise modify the oscillatoraccording to the description set forth above with respect to FIGS. 2a-2f.

Similar to the embodiment shown and described with respect to FIG. 3a,this un-narrowed output is then directed through a spectral filter 50,which is preferably one of the embodiments described in FIGS. 3b-3 d.Given the significant time (e.g., several nanoseconds) that it takes forthe beam to traverse the spectral filter 50, it is preferred to adjustthe arrival time of the filtered pulse to a second maximum of thedischarge current. To achieve this temporal adustment, an optical delayline is preferably inserted after the spectral filter 50. The delay linemay be one of those described at U.S. patent application Ser. No.60/130,392, which is assigned to the same assignee and is herebyincorporated by reference.

FIGS. 4b(i)-(iii) illustrate the electrical current through thedischarge gap, the intensity of the un-narrowed beam 76 and the output59 of the oscillator-amplifier system, each as a function of time. Thecurrent exhibits several cycles of oscillations, as shown in FIG. 4b(i).The optical pulse shown at FIG. 4b(ii) evolves towards the end of thefirst maximum (a) of current. The second maximum of electrical currentis separated from the first one by approximately 20 nanoseconds, thusproviding sufficient time for the beam 76 to traverse the spectralfilter 50 and additional optical delay line 78. This discussion withrespect to the timing of the successive maxima in the electricaldischarge current reveals how the additional optical delay line 78 maybe advantageously used to precisely tune the arrival time of the pulseat the chamber 70 (amplifier). The line-narrowed beam from the spectralfilter 50, whose temporal pulse shape is shown at FIG. 4b(iii), thusoverlaps the second maximum b of the electrical current shown at FIG.4b(i) of the amplifier and is amplified, and thus a line-narrowed beam59, i.e., substantially less than 1 pm, is output with sufficient powerto meet the first and second objects of the invention.

FIG. 5a shows the use of a line-narrowed oscillator followed by a poweramplifier made in a separate discharge chamber. Any of the embodimentsshown and described above including those discussed with respect to theexemplary embodiments, the patents and publications incorporated byreference,and the embodiments described with respect to FIGS. 2a-2 f canbe used to narrow the bandwidth of the oscillator. Examples of thepreferred line-narrowed oscillator 48 are set forth at FIGS. 5b-5 f.

The line-narrowed oscillator 48 schematically shown at FIG. 5(b) uses aprism beam expander 30 and grating 32, preferably as described in one orthe U.S. Pat. No. 5,559,816, 298 22 090.3 DE, U.S. Pat. Nos. 4,985,898,5,150,370 and 5,852,627 patents, each being incorporated by referenceabove. Alternatively, the Littman configuration may be used (seediscussion above with respect to FIG. 2b). As discussed above withrespect to the embodiments of FIGS. 2a-4 a, the additional apertures 34in the resonator reduce divergence of the beam and, therefore,advantageously increase the resolution of the wavelength selector(again, see the Ser. No. 09/130,277 application for details).

The embodiment shown in FIG. 5c utilizes multiple etalons 43 aswavelength selective elements (see FIG. 2d). The prism beam expander 30in combination with the apertures 34 helps to reduce the divergence ofthe beam in the etalons 43 thus improving resolution of the wavelengthselector. Additionally, this reduces the intensity of the beam at aparticular area of the surfaces of the etalons 43, thus extending theirlifetime.

FIGS. 5d-5 e show alternative arrangements that each include an RF ormicrowave excited waveguide laser as an oscillator. The arrangement ofFIG. 5d preferably includes a pair of RF-electrodes 80 and a waveguide82 preferably including a ceramic capillary filled with a laser activegas mixture. Any of the resonator configurations shown in FIGS. 2a-5 cmay be used in this embodiment, wherein the discharge chamber 2 isreplaced with the RF-excited waveguide arrangement shown in FIG. 5d.Features of the waveguide laser that may be used in the arrangement ofFIGS. 5d-5 e may be found at C. P. Christenson, Compact Self-ContainedArF Laser, Performing Organization Report Number AFOSR IR 95-0370; T.Ishihara and S. C. Lin, Theoretical Modeling of Microwave-PumpedHigh-Pressure Gas Lasers, Appl. Phys. B 48, 315-326 (1989); and Ohmi,Tadahiro and Tanaka, Nobuyoshi, Excimer Laser Oscillation Apparatus andMethod, Excimer Laser Exposure Apparatus, and Laser Tube, EuropeanPatent Application EP 0 820 132 A2, each of which is hereby incorporatedby reference. RF-excited lasers are commonly operated with a carbondioxide gas medium, e.g., as discussed in Kurt Bondelie “Sealed carbondioxide lasers achieve new power levels”, Laser Focus World, August1996, pages 95-100, which is hereby incorporated by reference.

The specific arrangement shown in FIG. 5d includes a prism beam expander30 and a grating 32 in Littrow configuration. A Littman configurationmay be used here (see FIGS. 2b and 2 f) including the grating 38 and HRmirror 40. A pair of apertures 34 are again included, particularly formatching the divergence of the resonator. A partially reflecting mirror36 outcouples the beam 20. An etalon outcoupler 46 may be used insteadof the mirror 36 (see FIGS. 2e-2 f)

The arrangement schematically shown at FIG. 5e is the same as that ofFIG. 5d, except that the grating is replaced with a one or more etalons43 and an HR mirror 44. A grating 32 or 38 may be used along with theetalons 43, and an etalon outcoupler 46 may be used instead of thepartially reflecting mirror 36.

An advantage of this RF-excited waveguide type of laser is its longpulse, which allows more efficient line narrowing, since the linewidthis approximately inversely proportional to the number of round trips ofthe beam in the resonator. Additionally, the RF-excited waveguide laserhas a small discharge width (on the order of 0.5 mm) which allows highangular resolution of the wavelength selector based on the prisms of thebeam expander 30 and the diffraction grating 32. This holds for both ofthe embodiments shown at FIGS. 5d-5 e.

FIG. 5f schematically shows another source of a narrow linewidth beamthat may be used in accordance with the present invention to serve asthe oscillator 48 in the embodiment of FIG. 5a. The arrangement of FIG.5f includes a solid state laser 85 with a third harmonic output at 355nm, such as diode pumped, Nd:YAG laser or other such type laser as maybe described, e.g., at U.S. Pat. No. 6,002,697, which is assigned to thesame assignee and is hereby incorporated by reference, or as may beotherwise known to one skilled in the art. The solid state laser 85, inturn, pumps a narrow linewidth tunable laser 86, such as a dye laser oroptical parametric oscillator, emitting, e.g., around 472.9 nm. This472.9 nm radiation is focussed into a gas cell 88 containing a mixtureof halide metal and inert gas, in order to produce a third harmonic beamat 157.6 nm. Such third harmonic generation in gases has been describedat: Kung A. H., Young J. F., Bjorklung G. C., Harris S. E., PhysicalReview Letters, v.29, Page 985 (1972); and Kung A. H., Young J. F.,Harris S. E, Applied Physics Letters, v.22 page 301 (1973), eachof whichis hereby incorporated by reference.

FIGS. 6a and 6 b schematically illustrate further embodiments wherein aportion of the discharge volume of a discharge chamber 2 is used as anoscillator with line narrowing, and the same discharge chamber 2 is usedas an amplifier 52. The arrangement of FIG. 6a is similar to that shownat FIG. 4a except that the linewidth of the beam 30 is narrowed withinthe resonator of the oscillator, and no spectral filter 50 is preferablyused. A spectral filter 50 may alternatively be used in addition to theline-narrowing optics of the oscillator of FIG. 6a. Again, theline-narrowing arrangement of the oscillator may be modified as setforth in any of the descriptions above (see particularly FIGS. 2a-2 f, 5c and 5 f), or as set forth in any of the patents, patent applicationsor publications incorporated by reference in this application, or asotherwise understood by one skilled in the art, to produce a narrowoutput beam 20 sufficient to meet the first object of the invention. Theoutput beam 20 from the oscillator is expanded by an external beamexpander 90, preferably comprising one or more prisms and alternativelycomprising a lens arrangement.

The expanded beam 92 is then directed through a delay line 78 (see the'392 application) to synchronize the pulse with the amplification maximaof the chamber 70, as described above. The optical delay line 78 servesto fine tune the arrival time of the optical pulse to the amplifiersection, similar to the embodiment shown and described with respect toFIGS. 4a-4 b(iii). The expanded beam 20 then advantageously fills asubstantial portion of the rest of the discharge cross section, and isamplified.

In the above embodiments, it is preferred to adjust the gas mixture inthe discharge chamber 2, 70 of the oscillator, to obtain the longestpossible pulse. Additionally, the waveform of the discharge current canbe modified by deliberately introducing an impedance mismatch of thepulse forming circuitry and discharge gap. The impedance mismatch leadsto a longer discharge time and thus, to a longer optical pulse. Thelower gain resulting from such modification means lower efficiency ofthe oscillator. However, in the embodiments discussed above, the amountof reduction in the output power of the oscillator is regained at theamplification stage.

While exemplary drawings and specific embodiments of the presentinvention have been described and illustrated, it is to be understoodthat the scope of the present invention is not to be limited to theparticular embodiments discussed. Thus, the embodiments shall beregarded as illustrative rather than restrictive, and it should beunderstood that variations may be made in those embodiments by workersskilled in the arts without departing from the scope of the presentinvention as set forth in the claims that follow, and equivalentsthereof.

In addition, in the method claims that follow, the steps have beenordered in selected typographical sequences. However, the sequences havebeen selected and so ordered for typographical convenience and are notintended to imply any particular order for performing the steps, exceptfor those claims wherein a particular ordering of steps is expressly setforth or understood by one of ordinary skill in the art as beingnecessary.

What is claimed is:
 1. A narrow band molecular fluorine laser system,comprising: an oscillator, comprising: a discharge chamber filled with alaser gas including molecular fluorine and a buffer gas, a plurality ofelectrodes within the discharge chamber connected to a discharge circuitfor energizing the laser gas, and a resonator including said dischargechamber and line-narrowing optics for generating a line-narrowed laserbeam having a wavelength around 157 nm, a linewidth less than 1 pm, andan energy below a predetermined energy; and an amplifier, comprising: agas-filled second discharge chamber at least including molecularfluorine; a second plurality of electrodes within the second dischargechamber connected to a same or a different discharge circuit forenergizing the gas within the second discharge chamber so that gain istimulated when the line-narrowed 157 nm beam generated by the oscillatoris passed through the energized gas of the amplifier, and wherein thelaser beam generated by the oscillator and passed through the amplifierhas an increased energy so that laser pulses output by the oscillatorand passed through the amplifier have an energy that is equal to orabove said predetermined pulse energy.
 2. The laser system of claim 1,wherein the line-narrowing optics include one or more etalons tuned formaximum transmissivity of a selected portion of the spectraldistribution of the beam, and for relatively low transmissivity of outerportions of the spectral distribution of the beam.
 3. The laser systemof claim 2, wherein the line-narrowing optics further include a beamexpander before the one or more etalons for expanding the beam incidenton the one or more etalons.
 4. The laser system of claim 3, wherein thebeam expander includes a plurality of beam expanding prisms.
 5. Thelaser system of claim 4, wherein the line-narrowing optics furtherinclude a grating for selecting a single interference order of the oneor more etalons, said selected interference order including saidselected portion of the spectral distribution of the beam.
 6. The lasersystem of claim 5, further comprising an aperture within the resonator.7. The laser system of claim 6, wherein the aperture is positionedbetween the discharge chamber and the beam expander.
 8. The laser systemof claim 5, further comprising a first aperture on one side of thedischarge chamber and a second aperture on the other side of thedischarge chamber.
 9. The laser system of claim 8, wherein the firstaperture is positioned between the discharge chamber and the beamexpander.
 10. The laser system of claim 4, further comprising anaperture within the resonator.
 11. The laser system of claim 10, whereinthe aperture is positioned between the discharge chamber and the beamexpander.
 12. The laser system of claim 4, further comprising a firstaperture on one side of the discharge chamber and a second aperture onthe other side of the discharge chamber.
 13. The laser system of claim12, wherein the first aperture is positioned between the dischargechamber and the beam expander.
 14. The laser system of claim 1, whereinthe line-narrowing optics include an output coupler tuned for maximumreflectivity of a selected portion of the spectral distribution of thebeam, and for relatively low reflectivity of outer portions of thespectral distribution of the beam.
 15. The laser system of claim 14,wherein the line-narrowing optics further include optics for selecting asingle interference order of the output coupler.
 16. The laser system ofclaim 14, wherein the line-narrowing optics further include a gratingfor selecting it single interference order of the output coupler. 17.The laser system of claim 16, wherein the line-narrowing optics furtherinclude a beam expander.
 18. The laser system of claim 17, wherein thebeam expander includes one or more prisms.
 19. The laser system of claim17, further comprising an aperture within the resonator.
 20. The lasersystem of claim 19, further comprising a second aperture, wherein thefirst and second apertures are positioned opposing sides of thedischarge chamber.
 21. The laser system of claim 14, wherein theline-narrowing optics further include a dispersive prism for selecting asingle interference order of the output coupler.
 22. The laser system ofclaim 21, wherein the line-narrowing optics further include a beamexpander.
 23. The laser system of claim 22, wherein the beam expanderincludes one or more prisms.
 24. The laser system of claim 22, furthercomprising an aperture within the resonator.
 25. The laser system ofclaim 24, further comprising a second aperture, wherein the first andsecond apertures are positioned opposing sides of the discharge chamber.26. The laser system of claim 21, wherein the line-narrowing opticsfurther include a beam expander.
 27. The laser system of claim 26,wherein the beam expander includes one or more prisms.
 28. The lasersystem of claim 26, further comprising an aperture within the resonator.29. The laser system of claim 28, further comprising a second aperture,wherein the first and second apertures are positioned opposing sides ofthe discharge chamber.
 30. The laser system of claim 14, wherein theline-narrowing optics further include an etalon for selecting a singleinterference order of the output coupler.
 31. The laser system of claim15, wherein the line-narrowing optics further include a beam expander.32. The laser system of claim 31, wherein the beam expander includes oneor more prisms.
 33. The laser system of claim 31, further comprising anaperture within the resonator.
 34. The laser system of claim 33, furthercomprising a second aperture, wherein the first and second apertures arepositioned opposing sides of the discharge chamber.
 35. The laser systemof claim 1, further comprising a highly reflective mirror and a grating.36. The laser system of claim 1, further comprising a grating, whereinthe grating comprises a Littrow diffraction grating.
 37. The lasersystem of claim 1, wherein said buffer gas includes neon forpressurizing the gas mixture sufficiently to increase the output energyfor a given input energy, said molecular fluorine being subject todepletion, and wherein said laser system further comprises: a gas supplysystem transferring molecular fluorine into said discharge chamber andthereby replenishing said molecular fluorine in the discharge chamber;and a processor cooperating with the gas supply system to control themolecular fluorine concentration within the discharge chamber tomaintain said molecular fluorine concentration within a predeterminedrange of optimum performance of the F2-laser.
 38. The laser system ofclaim 1, wherein said buffer gas includes neon for pressurizing the gasmixture sufficiently to increase the energy stability of the laser, saidmolecular fluorine being subject to depletion, and wherein said lasersystem further comprises: a gas supply system transferring molecularfluorine into said discharge chamber and thereby replenishing saidmolecular fluorine in the discharge chamber; and a processor cooperatingwith the gas supply system to control the molecular fluorineconcentration within the discharge chamber to maintain said molecularfluorine concentration within a predetermined range of optimumperformance of the F2-laser.
 39. The laser system of claim 1, furthercomprising an aperture within the resonator.
 40. The laser system ofclaim 39, wherein the aperture is positioned between the dischargechamber and the beam expander.
 41. The laser system of claim 1, furthercomprising a first aperture on one side of the discharge chamber and asecond aperture on the other side of the discharge chamber.
 42. Thelaser system of claim 41, wherein the first aperture is positionedbetween the discharge chamber and the beam expander.
 43. A narrow bandmolecular fluorine laser system, comprising: a discharge chamber servingas both an oscillator and an amplifier and being filled with a laser gasincluding, molecular fluorine and a buffer gas; a plurality ofelectrodes within the discharge chamber connected to a discharge circuitfor energizing the laser gas; a resonator including said dischargechamber and line-narrowing optics for generating a line-narrowed laserbeam having a wavelength around 157 nm, a linewidth less than 1 pm, andan energy below a predetermined energy; and extra-resonator optics forredirecting the been generated by and outcoupled from the oscillatorback into the discharge chamber, as an amplifier for increasing theenergy of the bean, so that laser pulses output by the oscillator andpassed between the plurality of electrodes upon being redirected intothe discharge chamber by said extra-resonator optics have an energy thatis equal to or above said predetermined energy.
 44. The laser system ofclaim 43, wherein said extra-resonator optics include an optical delayline for timing the entry of the beam back into the discharge chamberfor amplification at or near said time of maximum discharge current. 45.The laser system of claim 44, wherein said extra-resonator opticsinclude a beam expander for expanding the beam prior to re-entry intothe discharge chamber to enhance the amplification of the amplifier. 46.The laser system of claim 43, wherein the line-narrowing optics includeintra-resonator optics.
 47. The laser system of claim 46, wherein saidintra-resonator optics include one or more etalons tuned for maximumtransmissivity of a selected portion of the spectral distribution of thebeam, and for relatively low transmissivity of outer portions of thespectral distribution of the beam.
 48. The laser system of claim 47,wherein the intra-resonator optics further include a beam expanderbefore the one or more etalons for expanding the beam incident on theone or more etalons.
 49. The laser system of claim 48, wherein the beamexpander includes a plurality of beam expanding prisms.
 50. The lasersystem of claim 49, wherein the intra-resonator optics further include agrating for selecting a single interference order of the one or moreetalons, said selected interference order including said selectedportion of the spectral distribution of the beam.
 51. The laser systemof claim 50, further comprising an aperture within the resonators. 52.The laser system of claim 51, wherein the aperture is positioned betweenthe discharge chamber and the beam expander.
 53. The laser system ofclaim 50, further comprising a first aperture on one side of thedischarge chamber and a second aperture on the other side of thedischarge chamber.
 54. The laser system of claim 53, wherein the firstaperture is positioned between the discharge chamber and the beamexpander.
 55. The laser system of claim 50, wherein said extra-resonatoroptics include a beam expander for expanding the beam prior to re-entryinto the discharge chamber to enhance the amplification of theamplifier.
 56. The laser system of claim 46, wherein the intra-resonatoroptics include an output coupler tuned for maximum reflectivity of aselected portion of the spectral distribution of the beam, and forrelatively low reflectivity of outer portions of the spectraldistribution of the beam.
 57. The laser system of claim 56, wherein theintra-resonator optics further include optics for selecting a singleinterference order of the output coupler.
 58. The laser system of claim57, wherein the intra-resonator optics further include a beam expander.59. The laser system of claim 58, wherein the beam expander includes oneor more prisms.
 60. The laser system of claim 58, further comprising anaperture within the resonator.
 61. The laser system of claim 60, furthercomprising a second aperture, wherein the first and second apertures arepositioned opposing sides of the discharge chamber.
 62. The laser systemof claim 61, wherein said extra-resonator optics include a beam expanderfor expanding the beam prior to re-entry into the discharge chamber toenhance the amplification of the amplifier.
 63. The laser system ofclaim 56, wherein the intra-resonator optics further include a gratingfor selecting a single interference order of the output coupler.
 64. Thelaser system of claim 49, wherein the intra-resonator optics furtherinclude a beam expander.
 65. The laser system of claim 64, wherein thebeam expander includes one or more prisms.
 66. The laser system of claim64, further comprising an aperture within the resonator.
 67. The lasersystem of claim 66, further comprising a second aperture, wherein thefirst and second apertures are positioned opposing sides of thedischarge chamber.
 68. The laser system of claim 56, wherein theintra-resonator optics further include a dispersive prism for selectinga single interference order of the output coupler.
 69. The laser systemof claim 54, wherein the intra-resonator optics further include a beamexpander.
 70. The laser system of claim 69, wherein the beam expanderincludes one or more prisms.
 71. The laser system of claim 69, furthercomprising an aperture within the resonator.
 72. The laser system ofclaim 72, further comprising a second aperture, wherein the first andsecond apertures are positioned opposing sides of the discharge chamber.73. The laser system of claim 56, wherein the intra-resonator opticsfurther include an etalon for selecting a single interference order ofthe output coupler.
 74. The laser system of claim 59, wherein theintra-resonator optics further include a beam expander.
 75. The lasersystem of claim 74, wherein the beam expander includes one or moreprisms.
 76. The laser system of claim 75, further comprising an aperturewithin the resonator.
 77. The laser system of claim 62, furthercomprising a second aperture, wherein the first and second apertures arepositioned opposing sides of the discharge chamber.
 78. The laser systemof claim 46, wherein said extra-resonator optics include a beam expanderfor expanding the beam prior to re-entry into the discharge chamber toenhance the amplification of the amplifier.
 79. The laser system ofclaim 65, wherein said extra-resonator optics include a beam expanderfor expanding the beam prior to re-entry into the discharge chamber toenhance the amplification of the amplifier.
 80. The laser system ofclaim 67, wherein said extra-resonator optics include a beam expanderfor expanding the beam prior to re-entry into the discharge chamber toenhance the amplification of the amplifier.
 81. The laser system ofclaim 49, further comprising an aperture within the resonator.
 82. Thelaser system of claim 81, wherein the aperture is positioned between thedischarge chamber and the beam expander.
 83. The laser system claim 49,further comprising a first aperture on one side of the discharge chamberand a second aperture on the other side of the discharge chamber. 84.The laser system of claim 83, wherein the first aperture is positionedbetween the discharge chamber and the beam expander.
 85. The lasersystem of claim 84, wherein said extra-resonator optics include a beamexpander for expanding the beam prior to re-entry into the dischargechamber to enhance the amplification of the amplifier.
 86. The lasersystem of claim 69, wherein said extra-resonator optics include a beamexpander for expanding the beam prior to reentry into the dischargechamber to enhance the amplification of the amplifier.
 87. The lasersystem of claim 43, further comprising a highly reflective for and agrating.
 88. The laser system of claim 43, further comprising a grating,wherein the grating comprises a Littrow diffraction grating.
 89. Thelaser system of claim 43, wherein said buffer gas includes neon forpressurizing the gas mixture sufficiently to increase the output energyfor a given input energy, said molecular fluorine being subject todepletion, and wherein said laser system further comprises: a gas supplysystem transferring molecular fluorine into said discharge chamber andthereby replenishing said molecular fluorine in the discharge chamber;and a processor cooperating with the gas supply system to control themolecular fluorine concentration within the discharge chamber tomaintain said molecular fluorine concentration within a predeterminedrange of optimum performance of the F2-laser.
 90. The laser system ofclaim 43, wherein said buffer gas includes neon for pressurizing the gasmixture sufficiently to increase the energy stability of the laser, saidmolecular fluorine being subject to depletion, and wherein said lasersystem further comprises: a gas supply system transferring molecularfluorine into said discharge chamber and thereby replenishing saidmolecular fluorine in the discharge chamber; and a processor cooperatingwith the gas supply system to control the molecular fluorineconcentration within the discharge chamber to maintain said molecularfluorine concentration within a predetermined range of optimumperformance of the F2-laser.
 91. The laser system of claim 43, whereinsaid extra-resonator optics include a beam expander for expanding thebean prior to re-entry into the discharge chamber to enhance theamplification of the amplifier.